The main function of a switch is to close or open an electrical circuit. There are switches that switch the ground wire and switches that interrupt the supply current (plus).
Checking lamp switches and electric motors
Remove the corresponding switch.
Simple switches have only 2 terminals. In this case, voltage (+) is supplied to one of the terminals, and it appears on the second terminal after the switch is turned on. There are also switches with several terminals. With regard to these switches, you should find out from the electrical diagram which terminal should have voltage, if necessary, before turning on the ignition.
Use a tester to check if voltage is being supplied to the switch. If the lamp lights up, turn on the switch and check for voltage at the output terminal. If so, the switch is normal.
If there is no voltage at the input terminal, there is a break in the current supply wire. According to the electrical diagram, check the voltage supply and, if necessary, lay a new wire.
Checking the sensors-switches (alarms)
Sensor switches include, for example, oil pressure sensors and coolant or brake fluid level sensors.
Connect a tester or ohmmeter to the sensor terminals, removing the wires from them.
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Note: Sensors mounted on the engine block usually do not have a ground wire, since the sensor body, connected to the block, itself serves as a ground pole.
When the switch is closed, the device should show a short circuit. An ohmmeter is best suited for this purpose. When the switch is closed, the ohmmeter should show 0 Ohm, when open, ∞.
The fastest way to check the operation of the coolant and brake fluid level switches is to remove the wire from the switch with the ignition on and connect it to ground, for example, on the engine block. If the control lamp on the dashboard lights up, the switch is faulty.
A special case is the oil pressure switch: When the engine is not running, the contact is closed (the control lamp is on), the switch opens only at a certain pressure.